isci: fix a build warning

Use min_t to address:
drivers/scsi/isci/probe_roms.c: In function ‘isci_get_efi_var’:
drivers/scsi/isci/probe_roms.c:241: warning: comparison of distinct pointer types lacks a cast

Reported-by: David Milburn <dmilburn@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>